Industry has generally accepted widespread use of a slurry polymerization process to produce butyl rubber in a diluent, commonly methyl chloride. Typically, the polymerization of isoolefins such as isobutylene with any comonomers uses methyl chloride at low temperatures, generally lower than −90° C., as a diluent for a reaction mixture. Methyl chloride is used for a variety of reasons, including that it dissolves monomers and aluminum chloride catalyst but not polymer product. Methyl chloride also has suitable freezing and boiling points to permit, respectively, low temperature polymerization and effective separation from the polymer and unreacted monomers.
Commercial reactors typically used to make butyl rubber slurries are well mixed vessels of greater than 10 to 30 liters in volume with a high circulation rate provided by pump impellers. The polymerization and the pumps both generate heat, which is removed by heat exchangers to keep the slurries cold. The slurries are circulated through heat exchanger tubes. The product slurry is generally transferred from the butyl reactor to a quench drum or tank where it is mixed with a quench fluid, usually steam and/or hot water, to terminate any further polymerization and remove the diluent.
The polymer usually has a lower density than the diluent, and a reactor overflow line is used to transfer the polymer slurry from the reactor. The overflow transfer line is typically in the shape of an inverted U which can accommodate the thermal expansion between the chilled polymerization reactor (−90° C. or below) and a flash tank that is generally operated at a relatively warmer temperature ranging from the boiling point of the diluent up to the boiling point of water, e.g. from +40° to 100° C. A schematic illustration of a prior art transfer line 2 connecting a slurry polymerization reactor 4 and a flash tank 6 can be seen in FIG. 1. The transfer line 2 typically terminates at a quench nozzle 7 wherein it is mixed with steam and/or hot water 8 which can be withdrawn and pumped from the lower end of the flash tank 6.
Reactor overflow transfer lines have a tendency to plug during polymer production cycles when using methyl chloride diluent. In methyl chloride diluent, the polymer particles tend to contain dissolved diluent and can be soft with a tendency for particles to stick together and to reactor surfaces, i.e. the particles are “sticky” and thought to cause transfer line plugging by agglomeration of particles and adhesion to the surfaces in the transfer line. Typically with methyl chloride diluent, the transfer line can be unplugged using a steaming practice which is thought to evaporate a thin film of methyl chloride on the internal surfaces of the line and/or to expel methyl chloride from the polymer particles. Elaborate steam sparging lines and condensate collection systems (not shown), including steam jacketing of the transfer line, have been devised for unplugging or preventing plugging of the transfer lines. The plug can often be released in this manner and pressured out of the transfer line, due to the soft nature of the rubber particles when using methyl chloride.
More recently, the polymerization of isobutylene and other monomers in hydrofluorocarbon (HFC) diluents, such as tetrafluoroethane, has been disclosed. The utilization of HFC's in diluents or blends of diluents has created new polymerization systems that reduce particle agglomeration, and also can eliminate or reduce the amount of chlorinated hydrocarbons such as methyl chloride in polymerization systems. Such new polymerization systems reduce particle agglomeration and fouling in the reactor without having to compromise process parameters, conditions, or components and/or without sacrificing productivity/throughput and/or the ability to produce high molecular weight polymers. HFC's are chemicals that are currently used as environmentally friendly refrigerants because they have a very low (even zero) ozone depletion potential, and also typically have low flammability particularly as compared to hydrocarbons and chlorinated hydrocarbons.
Some polymerization media, processes, reactors and systems that can employ HFC's are disclosed in the following commonly assigned patent references: WO2004058827; WO2004058828; WO2004058829; WO2004067577; WO2006011868; US2005101751; US2005107536; US2006079655; US2006084770; US2006094847; US2006100398; and US2006111522.
When using an HFC, the transfer line also has a tendency to plug and, unlike methyl chloride slurries, can not be easily cleared with the application of steam and pressure. This is surprising because the HFC slurry particles are not as sticky as the methyl chloride slurry particles, and the stickiness of polymer particles is widely believed to be a major contributing factor to transfer line plugging. On the other hand, the HFC slurry particles are hard and have a tendency to form very hard plugs which cannot be removed by steaming.
It is estimated that, regardless of the diluent used, transfer line plugging has been a significant source of down time for butyl reactors used in the industry for more than half a century. Yet, very little research and development has been forthcoming on the subject of inhibiting or eliminating transfer line plugging events. One approach, that demonstrates both the difficulty of the problem and the overly complicated attempts which operators are willing to undertake in order to try to solve the problem, involves the use of a twin screw extruder in the transfer line as disclosed in U.S. Patent Pub. No. US2005187366.
There is clearly a long-felt and unsatisfied need in the art for improved transfer line systems and methods for use with butyl reactors and similar processes, that are simple in design and operation, and effectively avoid the frequent occurrence of plugging.